This invention relates in general to a circuit for measuring the duration of a pulse train and more particularly to such a circuit for providing fading compensation radio receivers.
A circuit for measuring the duration of a pulse train has been disclosed in the journal "Elektronik", 1966, pp. 143 to 146, FIG. 9, page 145. In this circuit a counter, during the period to be measured, or an integer multiple thereof serving as the measuring period, counts pulses which are derived from an oscillator serving as the time base. A gate or sequence-control circuit controlled by the pulse train to be measured, is used to control the counter in such a way that the aforementioned oscillator pulses are counted only during the measuring period. The counter reading (count) is made visible with the aid of an indicating or display arrangement for which it is possible to use the most various types of nowadays customary electronic indicating or display means (digital display tubes, LED's, seven-segment displays, etc.).
In particular fields of practical application of such period or duration measuring circuits it is also customary to provide a memory (storage device) beween the counter output and the indicating arrangement, in which the respective count is temporarily stored. This arrangement is customary, for example, with traffic decoders which, among others, also decode the service area identification signal, German Patent Application DE-OS No. 25 18 104. The general principles for designing a traffic radio system may be found in the German technical journal "Funkschau", 1974, pp. 535 to 538.
In conventional circuits for measuring the duration of a pulse train, for example, when designed as service area identification decoders for traffic radio signals, or when designed as a digital voltmeter, it is necessary to make both the measuring and the display insensitive with respect to interferences to which the pulse train to be measured may be subjected. These interferences may become noticeable, on one hand, in that individual pulses are missing and, on the other hand, also in that additional pulses are superposed, or else in that the pulse periods vary. The conventional arrangement according to DE-OS No. 25 18 104, in which instead of the pulses of an oscillator, the oscillations of the 57-kHz identification are counted in the counter following a corresponding frequency division, eliminates these interferences to a certain extent, in that a predetermined count range is used for indicating the same service area identification symbol, that is, within the count range between 16 and 20, the service area "A" is displayed. In this, the rated display is e.g. at the count 18, so that per measuring period it is possible to tolerate a variation of .+-.2 pulses of the divided 57-kHz identification oscillation.